12.3 The Periodic Table The periodic table organizes the elements according to how they combine with other elements (chemical properties). The periodic table is organized in order of increasing atomic number.

12.3 The Periodic Table The periodic table is further divided into periods and groups. Each horizontal row is called a period. Each vertical column is called a group.

12.3 The Periodic Table All the elements in Group 1 of the periodic table form similar compounds. The metals lithium, sodium, and potassium all form compounds with a ratio of 2 atoms of the metal to 1 atom of oxygen.

12.3 Atomic Mass The mass of individual atoms is so small that the numbers are difficult to work with. To make calculations easier, scientists use the atomic mass unit (amu). The atomic mass of any element is the average mass (in amu) of an atom of each element.

12.3 Atomic Mass Atomic masses differ from mass numbers because most elements in nature contain more than one isotope.

12.3 Atomic Number Remember, the atomic number is the number of protons all atoms of that element have in their nuclei. If the atom is neutral, it will have the same number of electrons as protons.

12.3 Groups of the periodic table The first group is known as the alkali metals. The alkali metals are soft and silvery in their pure form and are highly reactive. This group includes the elements lithium (Li), sodium (Na), and potassium (K).

12.3 Groups of the periodic table The Group Two metals include beryllium (Be), magnesium (Mg), and calcium (Ca). They also bond easily with oxygen.

12.3 Halogens The halogens tend to be toxic gases or liquids in their pure form. Fluorine (F), chlorine (Cl), and bromine (Br) form salts when the bond with alkali metals.

12.3 Noble Gases The noble gases, including the elements helium (He), neon (Ne), and argon (Ar). These elements do not naturally form chemical bonds with other atoms and are almost always found in their pure state.

12.3 Transition metals In the middle of the periodic table are the transition metals, including titanium (Ti), iron (Fe), and copper (Cu). These elements are usually good conductors of heat and electricity.